K S 3 G E O G R A P H Y T E A C H E R S ’ T O O L K I T : T H E R O L E O F S T O N E S
© Geographical Association, 2014
Physical, chemical and
biological weathering
Weathering is the decay and disintegration of rock by the
action of sun, rain, ice, wind and living things to form
soil. There are three main types of weathering;
• Physical weathering caused by changes in
temperature and pressure
• Chemical weathering caused by the action of water
and chemicals
• Biological weathering caused by the action of plants
or animals. It can be either physical or chemical (see
below)
Physical weathering
Types of physical weathering include:
Thermal stress – happens when rock repeatedly warms
up and cools down with changes in temperature. This is
most likely to happen in deserts that are extremely hot
by day and cold by night. The surface of the rock
expands and contracts causing stress within the rock,
making it break up.
Freeze-thaw – happens when water finds its way into a
crack in a rock. In very cold weather, especially on
mountains, the water may freeze. When water freezes it
expands, like a wedge, forcing the crack to widen. If this
happens repeatedly the rock will eventually split into
fragments. This is why you often see lots of broken
pieces of rock at the bottom of a mountain.
Chemical weathering
Types of chemical weathering include:
Carbonation – happens when rainfall absorbs carbondioxide
from the atmosphere to form a weak acid called
carbonic acid. Sometimes, in polluted air, the rain may
absorb other gases, such as sulphur dioxide, to form an
even stronger acid. Carbonic acid reacts with calcium
carbonate in limestone or chalk to dissolve it. In this
way, the rock slowly decays.
Oxidation – happens when oxygen in the atmosphere
reacts with the minerals in a rock to form a new
chemical. The most common example is when iron is
turned into iron oxide (rust). As a result the rock changes
colour, becoming browner, and begins to crumble. This
is most likely to happen when the rock is wet.
Biological weathering
Biological weathering can be either physical or chemical,
or both.
Physical – happens when plant roots find their way into
the cracks in a rock and begin to grow. The roots force
the crack to widen and eventually cause the rock to
break apart. Small animals, such as worms, also help to
break up the rocks.
Chemical – happens when small plants called lichens – a
combination of fungi and algae – grow on the surface of
a rock. They produce chemicals that slowly make the
surface of the rock disintegrate.
LESSON 7: Information Sheet 6
Photo©JasonHollinger.